The invention relates to the field of micro-electronics and more specifically, to the sector for fabricating microcomponents, especially those intended to be used in radio frequency applications. More specifically, it relates to microcomponents such as microinductors or microtransformers equipped with a magnetic core allowing the operation at particularly high frequencies.
As is known, electronic circuits used for radio frequency applications, especially such as mobile telephony, comprise oscillating circuits including capacitors and inductors.
Given the trend toward miniaturization, it is essential that microcomponents such as microinductors occupy an increasingly small volume, while keeping a value of inductance which is high enough and a high quality coefficient.
Moreover, the general trend is toward increasing operating frequencies. Thus, mention may be made by way of example of the frequencies used in the new UMTS standards of mobile telephony, which are in the region of 2.4 gigahertz, in comparison with the frequencies of 900 and 1800 megahertz used for the GSM standard.
The increase in operating frequencies poses problems relating to the behavior of magnetic cores of microinductors.
This is because, in order to obtain a good quality factor, an increase in the inductance of the microinductor is generally sought. To this end, magnetic materials are chosen, the geometry and dimensions of which enable the greatest possible permeability to be obtained.
However, given the phenomena of gyromagnetism, it is known that the permeability varies according to the frequency, and more specifically, that there is a resonance frequency beyond which an inductor has capacitative behavior. In other words, a microinductor absolutely must be used at frequencies below this resonance frequency.
However, increasing the frequencies of use therefore comes up against the phenomenon of gyromagnetic resonance, which, for a given geometry, limits the frequency range in which the inductor can be used in an optimal manner.
A problem which the invention proposes to solve is that of the limitation of the frequency of use inherent to the existence of a phenomenon of gyromagnetism.
The aim of the invention is therefore an inductive microcomponent, such as a microinductor or microtransformer, comprising a metal winding having the shape of a solenoid and a magnetic core made of ferromagnetic material positioned at the center of the winding.
According to the invention, the core of this microcomponent consists of several sections separated by cutouts oriented perpendicularly to the main axis of the solenoid.
In other words, the magnetic core does not form a monolithic part aligned along the axis of the solenoid, but on the contrary it is segmented in the direction of the solenoid.
The fractionation of the magnetic core causes a decrease in the magnetic permeability of each section, and therefore a decrease in the value of inductance of the microcomponent. Nevertheless, it has been noticed that this drawback is compensated for by the increase in the maximum frequency to which the microcomponent keeps its inductive behavior.
The gyromagnetic resonance frequency is determined by the Landau-Lifschitz equation which follows:             1      γ        ⁢                  ∂                  M          →                            ∂        t              =            M      →        ⩓                  H        →            +                        α                      γ            ⁢                          xe2x80x83                        ⁢            M            ⁢                          xe2x80x83                        ⁢            s                          ⁢                              ∂                          M              →                                            ∂            t                                ⩓          M      →      
in which:
M is the magnetic moment,
H is the magnetic field in which this moment is immersed,
xcex3 is the gyromagnetic constant,
xcex1 a is the damping factor.
In order to determine the permeability along the difficult axis of the ferromagnetic material, which corresponds to the main axis of the solenoid, we need to determine the various magnetic fields to which the material is subject. Thus, when a material of a given shape is immersed in a magnetic field (Hext), the magnetizations have a tendency to align themselves.
The neutrality of the material is therefore lost, charges appear which create a field opposing the external field, thus decreasing the resultant internal field (Hint). The field opposing the external field is generally called a xe2x80x9cdemagnetizing fieldxe2x80x9d (Hd), and depends strongly on the geometry. More specifically, the demagnetizing field coefficient is called N such that:
{overscore (H)}d=xe2x88x92N{overscore (M)}
This coefficient depends only on the geometry. This demagnetizing field, created by magnetic components in the direction of the difficult axis decreases the resulting internal field and therefore opposes the passage of the flux lines. In other words, this demagnetizing field has the consequence of reducing the permeability.
Thus, by taking into account this model, it is possible to solve the Landau-Lifschitz equation in order to determine the value of the permeability along the difficult axis. As is known, the magnetic permeability is a complex quantity in which the real part represents the effective permeability, while the imaginary part represents the losses. Thus, solving these equations gives the values of the real part (xcexcxe2x80x2) and of the imaginary (xcexcxe2x80x3) as a function of the frequency, of N and of the intrinsic properties of the material.
The resonance frequency, for which the value of xcexcxe2x80x3 is maximum, is as follows:       f    res    =            γ              2        ⁢        π              ⁢                            (                                    H              k                        +                                          N                ·                4                            ⁢              π              ⁢                              xe2x80x83                            ⁢                              M                s                                              )                ⁢                  xe2x80x83                ⁢                  (                                    H              k                        +                                          4                ·                π                            ⁢                              xe2x80x83                            ⁢                              M                s                                              )                    
in which:
N is the demagnetizing field coefficient,
xcex3 is the gyromagnetic constant,
Hk is the value of the saturation magnetic field, and
Ms is the value of the magnetic moment at saturation.
It is therefore found that the resonance frequency increases with the demagnetizing field coefficient N. For parallelepipedal geometries, the demagnetizing field coefficient depends on:
the length of the parallelepiped measured along the difficult axis, that is to say, along the solenoid axis,
the thickness of the parallelepiped,
the width along the easy access.
Thus, by virtue of the geometry chosen for the core according to the invention, the magnetizing field coefficient is considerably higher than for a monolithic core occupying the whole length of the solenoid. It follows that the demagnetizing field is also stronger and that the magnetic permeability along the difficult axis is smaller.
In return, the resonance frequency for the gyromagnetic effect is higher, which makes it possible to use the microinductor or the microtransformer at higher frequencies.
Advantageously, in practice, it has been determined that the coupling phenomena between the various sections of the core are negligible or have little effect when the width of the cutouts separating the sections of the core, measured in the direction of the solenoid axis, is greater than four times the thickness of the core.
When this width is considerably less than this value, the magnetic coupling phenomena between the various sections contribute to giving the set of sections a behavior which is similar to that of a monolithic core, with the already stated limitation relating to the resonance frequency. Conversely, when the separation of the sections is too great, the value of the inductance reduces because of the reduction in the magnetic volume.
Advantageously, in practice the thickness of the core may be between 0.1 and 10 micrometers. Indeed, it has been found that it is possible to overcome induced current phenomena, which are correspondingly greater the higher the frequency of use, by limiting as much as possible the thickness of each section of the magnetic core.
However, in order to keep a high enough value of permeability, it is possible, in a particular embodiment of the invention, to make the core from several superimposed magnetic layers, each one having a limited thickness.
In practice, the core can be made from materials chosen from the group comprising iron, nickel, cobalt, zirconium or niobium based alloys.
Microinductors having a minimum series resistance and therefore a particularly high quality factor are obtained by making the solenoid from electrolytic copper, which can be deposited on an insulating substrate such as quartz or glass. The solenoid can also be deposited on a conducting or semi-conducting substrate, with the interposition of an insulating layer between this substrate and the solenoid.